docs: various fixes to the graphql tutorial (#2965)

* doc/md: fix resolver diff and isolation example

* doc/md: fix updateTodo input field

* doc/md: add missing extension and fix paths

Without entgql.WithSchemaGenerator entc.go fails to run.

* doc/md: add file titles

Note that the where:TodoWhereInput parameter is pre-filled in the
generated ent.graphql file.

* doc/md: add context to conjunction/negation example

Only add for the first example to provide a lead to readers.

doc/md/tutorial-todo-gql-tx-mutation.md

@@ -37,16 +37,12 @@ srv := handler.NewDefaultServer(todo.NewSchema(client))
 
 2\. Then, in the GraphQL mutations, use the client from context as follows:
 ```diff title="todo.resolvers.go"
-func (mutationResolver) CreateTodo(ctx context.Context, todo TodoInput) (*ent.Todo, error) {
+}
++func (mutationResolver) CreateTodo(ctx context.Context, input ent.CreateTodoInput) (*ent.Todo, error) {
 +	client := ent.FromContext(ctx)
-+	return client.Todo.
--	return r.client.Todo.
-		Create().
-		SetText(todo.Text).
-		SetStatus(todo.Status).
-		SetNillablePriority(todo.Priority). // Set the "priority" field if provided.
-		SetNillableParentID(todo.Parent).   // Set the "parent_id" field if provided.
-		Save(ctx)
++	return client.Todo.Create().SetInput(input).Save(ctx)
+-func (r *mutationResolver) CreateTodo(ctx context.Context, input ent.CreateTodoInput) (*ent.Todo, error) {
+-	return r.client.Todo.Create().SetInput(input).Save(ctx)
 }
 ```
 
@@ -54,17 +50,17 @@ func (mutationResolver) CreateTodo(ctx context.Context, todo TodoInput) (*ent.To
 
 If you'd like to tweak the transaction's isolation level, you can do so by implementing your own `TxOpener`. For example:
 
-```go
+```go title="cmd/todo/main.go"
 srv.Use(entgql.Transactioner{
 	TxOpener: entgql.TxOpenerFunc(func(ctx context.Context) (context.Context, driver.Tx, error) {
 		tx, err := client.BeginTx(ctx, &sql.TxOptions{Isolation: sql.LevelRepeatableRead})
 		if err != nil {
 			return nil, nil, err
 		}
-		ctx = NewTxContext(ctx, tx)
-		ctx = NewContext(ctx, tx.Client())
+		ctx = ent.NewTxContext(ctx, tx)
+		ctx = ent.NewContext(ctx, tx.Client())
 		return ctx, tx, nil
-	})
+	}),
 })
 ```